1. Field of the Invention
The present invention relates to a nonlinear optical element for use in an optical information processing system, e.g. an optical computer. The present invention is further concerned with a method of using such a nonlinear optical element as an optical bistable element for a logic element or a memory element, as in such optical computer or the like.
2. Prior Art
FIG. 16 is a conceptual diagram illustrating a conventional optical bistable device which has been referred to in David A. B. Miller et al. in IEEE Journal of Quantum Electronics, Vol. QE-21, No. 9 (1985), pp. 1462-1476. In the same figure, the reference numeral 1 denotes an AlGaAs/GaAs multiple quantum well layer. On each of the upper and lower surfaces of the multiple quantum well layer 1 there is disposed a non-doped (i-type) AlGaAs layer 2. A p-type AlGaAs layer 5 is formed on the non-doped AlGaAs layer 2 which is disposed on the upper surface side of the multiple quantum well layer 1, while an n-type AlGaAs layer 6 is formed on the lower surface of the non-doped AlGaAs layer 2 which is disposed on the lower surface side of the layer 1. On the upper surface of the p-type AlGaAs layer 5 and on the lower surface of the n-type AlGaAs layer 6 there are provided upper electrodes 7a and lower electrodes 7b, respectively. An external power source 31 is connected between electrodes 7a and 7b, and an external resistor 30 having a resistance value of R is connected between the electrode 7a and the external power source 31.
FIG. 17 is a diagram for explaining the operation of the device illustrated in FIG. 16, and FIG. 18 is a characteristic diagram showing a relation between an incident light intensity P.sub.in and a transmitted light intensity P.sub.out in that device.
The operation will now be described. In the conventional optical bistable device, which is constructed as above, a reverse bias V.sub.ex is applied to the p-i-n photodiode from the external power source 31 via the external resistor 30 having the resistance R. The multiple quantum well layer 1 located in the interior of the photodiode has a sharp absorption spectrum due to excitation absorption corresponding to a transition between quantum levels. The peak of the absorption spectrum can be shifted by changing the internal electric field. That is to say, the absorption factor with respect to the wavelength of a certain incident light in the multiple quantum well layer 1 has a peak in a certain internal electric field.
Light incident on the photodiode is absorbed in the multiple quantum well layer 1 and there flows a photocurrent I proportional thereto. The solid curve in FIG. 17 represents a relation between a voltage V which is applied to the photodiode and a light absorption factor S of this photodiode under the condition of no external resistor. As can be seen from this curve, the light absorption factor has some peaks at specific values of voltage V applied to the photodiode. In the external resistor 30 included in the circuit of FIG. 16, there occurs a voltage drop for the photocurrent I and the (internal) voltage V applied to the photodiode changes from V.sub.ex to V.sub.ex -IR. That is, EQU V=V.sub.ex -IR (1)
On the other hand, the relation between the photocurrent I and the absorption factor S is obtained as: EQU I=.alpha.SP.sub.in ( 2)
where .alpha. is a proportion constant and P.sub.in represents an incident light intensity.
From the equations (1) and (2). the absorption factor S is obtained as: EQU S=(V.sub.ex -V)/.alpha.RP.sub.in
Thus, in the relation to the voltage V, the absorption factor S can be represented by a straight line whose gradient becomes smaller with increase of the incident light intensity P.sub.in. The straight broken lines A to D in FIG. 17 represent this relation in the equation (2) for different incident light intensities P.sub.in, wherein in one hand the straight line A is obtained when the incident light intensity P.sub.in is the lowest, and on the other hand the straight line D is obtained at the highest incident light intensity P.sub.in. Actual operation points under the incidence of light are represented by intersection points between such straight lines and the foregoing curve.
The straight lines A and D each have only one intersection point with respect to the curve representing the V-S relation, while between the straight lines B and C there are obtained three intersecting points. Of these three intersection points, the central point is an unstable point. The other two, however, are stable points. That is, bistable characteristics are shown between the straight lines B and C. As illustrated in FIG. 18, two different optical outputs, i.e. bistable characteristics, are obtained in a certain incident light intensity P.sub.in range.
In the above construction of the conventional optical bistable device, it is necessary to provide an external circuit including external resistor and power source. Further, feedback is performed using an electric current which has been taken out to the exterior. Thus, when it is desired to obtain bistable characteristics between plural incident lights and plural transmitted lights, there has been a complicated problem that element isolation is required and an external circuit is needed in each of the isolated regions made by the element isolation.